pub fn main() {
    let str1 = String::from("aaa");
    let str2 = String::from("bbb");

    let str3 = str1.clone() + &str2;
    let str3 = format!("{}{}", str1, str2);

    println!("str1:{}", &str1);
    println!("str2:{}", &str2);
    println!("str3:{}", &str3);

    // =====================================
    let me = String::from("我");
    println!("{}", &me.len());
    println!("{}", &me[0..3]);
}